1. Sensation & Perception
Creativity is the ability to see relationships where none exist. Thomas Disch
And now here is my secret, a very simple secret:
It is only with the heart that one can see rightly;
what is essential is invisible to the eye.
Atoine de Saint-Exupéry

2. Why Sensation & Perception?
TN Psychology Standards 6.7 understand the relationship between the brain, sensory perceptions, and behavior. 6.9 analyze the role of perceptions in the psychological growth and development of humans.
OUR objectives:
To distinguish between sensation and perception using all five senses
To identify and explain how the parts of the body work together to create sensory experiences
To use our senses to explore our environments

3. So who is on top – the people pointing over the fence or the guys on the ladder??

5. They’re both the same!!

7. Hint: The duck is looking left and the rabbit is looking right.

8. Selective Attention
Focusing conscious awareness on a particular stimulus to the exclusion of others is called selective attention.
E. G. Boring’s ambiguous figure is an example of selective attention. What do you see?

9. What’s the difference?
Sensation: Special receptors in the sense organs – the eyes, the ears, etc. – are activated, allowing various forms of outside stimuli
Perception: Organizing sensations into meaningful patterns
Transduction: the process of converting outside stimuli (i.e. light) into neural activity
Stimulus: Form of energy that can affect sense organs
We’re moving now into an area of psychology that studies how we take in information about the world around us.
This chapter, sensation and perception, deals with two different facets of this phenomenon – what we decide to notice and pay attention to and then what we do with that information. While the terms get used interchangeably, sensation actually refers to the process of detecting a stimulus while perception involves using that sensory information to form a meaningful pattern. It’s the difference between noticing that there are fluctuations in the color of the screen and turning those fluctuations into letters and words.
A stimulus can be anything that attracts attention by a sensory organ.

10. Sensation Psychophysics Sensory receptors
Is the study of the relationship between stimuli & our psychological response to them
Sensory receptors
Detect stimuli & convert energy into neural impulses
Receptors are designed to serve very specific functions. All sense organs have their own receptors.
Psychophysics is the branch of psychology that studies the relationships between stimuli and our psychological response to them. This relationship is not a simple one-to-one relationship.
For example, have you ever set the volume on your alarm clock, thinking that it was reasonable, only to feel alarmed by the noise level of the alarm when you were awakened by it? Or have you ever started the car and had to quickly turn down the volume on the radio from where it was set last time you were in the car? The level of energy (I.e., loudness) of the radio hasn’t changed (the volume know remains in the same place as when you last had it on), but your perception of the loudness has changed drastically!
Although we can measure the physical stimulus very precisely, its effect on the observer is not so simple.
There are five major sensory receptors, which detect stimuli and, by converting energy into those action potentials we’ve talked about, create neural impulses that send messages to the brain. We’ll go over the visual system, auditory system, the skin senses, smell, and touch.

11. The Eye (Click on the title for an interactive eye diagram.)

12. Parts of the Eye Cornea Pupil Iris
Light is initially focused by this transparent covering over the eye
Pupil
Light enters the eye through this opening
Iris
Muscle connected to the pupil that changes its size to let in more or less light
Everyone has a unique iris (thus it is a new security technique being employed by some organizations)
We’re now going to walk through the various parts of the eye.
Light rays from the outside world first pass through the cornea, a clear curved membrane or ‘window’. The cornea bends light so that it is sharply focused within the eye. Abnormalities in the shape of the cornea cause astigmatism.
The pupil is the small, round hole in the iris through which light passes. The iris causes the pupil to dilate (enlarge) under dim viewing conditions to let in more light and to contract (shrink) under brightness to let in less light.
Next comes the ring-shaped iris, which gives the eye its color. The iris is a muscle that is controlled by the autonomic nervous system (see, this stuff is coming back to haunt you!). Its function is to regulate the sized of the pupil.

13. Parts of the Eye Lens Retina
This flexible disk under the cornea focuses light onto the back of the eye
Accommodation
Flexibility of the lens allows eye muscles to adjust light from objects at various distances away
Retina
Light reflected from the lens is received by this sheet of tissue at the back of the eye
Contains the receptors that convert light to nerve impulses
Behind the pupil, light continues through the lens, another transparent structure whose function is to fine-tune the focusing of the light. The lens brings an image into focus through changing its shape, in a process called accommodation. Specifically, the lens becomes more rounded for focusing on nearby objects and flatter for more distant objects (the cornea, which has a fixed shape, cannot make these adjustments for different distances). With age, the lens loses much of its elasticity and keeps the flatter shape appropriate for viewing at a distance. As a result, many middle-aged people start to need glasses for reading or bifocals with a near-vision portion in the lower part of the glass.
The retina is a multi-layered screen of cells that lines the back inside surface of the eyeball. It is one of the most fascinating tissues in the body – both because of its function, which is to transform patterns of light into images that the brain can use, and because of its structure, which illustrates many basic principles of neural organization. In an odd twist of nature, the image projected on the retina is upside down. That is, light from the top part of the visual field stimulates photoreceptor cells in the bottom part of the retina and vice versa.
The retina has aptly been called an extension of the brain. It has several relatively transparent layers and contains 130 million photoreceptor cells that convert light energy into neural activity.

14. How we see color: Cones
Retinal cells that respond to particular wavelengths of light, allowing us to see color
Most of our cones are located on the fovea, which gives us the sharpest resolution of visual stimuli
3 types of cones, each sensitive to different light frequencies
Cones are shorter, thicker more tapered cells that are sensitive to color under high levels of illumination. Cones are densely clustered in the center of the fovea, the pinhead-size center of the retina. Unlike the rest of the retina, the fovea contains only cones, and the ratio of rods to cones increases in the outer edges of the rods.
We have about 7 million cones. This enables us to see colors under normal lighting. Owls and other nocturnal animals don’t have any cones – they can see in night but it is only in black and white.

15. How we see color… Opponent-process theory Which is correct?
Trichromatic theory (the color wheel)
3 types of sensors
Red, blue & green receptors combine to
to form millions of color combinations.
Opponent-process theory
Receptors respond to pairs of colors
The pairs are white-black / red-green / yellow-blue.
Which is correct?
Both!
An afterimage is a sensation that persists after prolonged exposure to a stimulus. What did you see before? In situations such as this one, staring at a green image leaves a red afterimage, yellow leaves a trace of blue and black leaves white.
Putting the pieces together, Hering proposed the opponent-process theory of color vision. According to this theory, there are three types of visual receptors, and each is sensitive to a pair of complementary or opponent colors. One type reacts to the colors blue and yellow, a second type detects red and green, and a third type detects variations in brightness ranging from black to white. Within each pair of red-green, blue-yellow, and black-white receptors, some parts fire more to one color whereas other parts react to its opposite. That’s why we never see bluish yellow or reddish green but we might see bluish green and reddish yellow. While seeing one color at a specific spot on the retina, you cannot also see its opposite color on the same spot.
Trichromatic theory argues that the eye has three kinds of color sensors (cones), with each sensor responding maximally to a distinct range of wavelength. Color vision arises from the combinations of neural impulses from these three different kinds of sensors. A lot of researchers believe that the corresponding wavelengths here map onto blue, green, and red. For example, in presenting a yellow (580 nm) stimulus to a participant, you would most likely see a large amount of firing of the green (530 nm) and red (650 nm) cones, with little to no firing of the blue (460 nm) cones.
Other researchers argued that some color combinations don’t make sense – red and green mixed together don’t make yellow; it looks more like mud. But, in addition to the fact that mixing physical colors before the wavelengths get to the retina is a different process than the cones firing when the resulting wavelength reaches the retina, sensing color on the retina from various wavelengths then gets sent to the brain for perception via opponent-process cells in the brain. Thus, the opponent-process theory was offered as a complimentary theory to the trichromatic theory. This theory states that if a color is present, it causes cells that register it to inhibit the perception of an opposing color.
The opponent-process theory proposes six psychologically primary colors, which are assigned by pairs to three kinds of receptors: white-black receptors, red-green receptors, and yellow-blue receptors. If white is present, then the perception of black is inhibited, if red is present then green is inhibited, and if yellow is present then blue is inhibited.
Inhibitions, along with various mixtures of these primary light wavelengths, more readily explain the complex range of colors that we experience after sensations of colors are sent from the retina to the brain’s opponent-process receptor cells.
To clarify:
Trichromatic theory and opponent-process theory work at two different levels, specifying different receptor cells at different stages in the process.
When you are looking at black words on a white page, you see both black and white because you are stimulating many white-black receptors on your retina. You see a black letter with one white-black receptor (and doing so inhibits seeing white in that particular receptor) and you see the background white with other white-black receptors (and doing so inhibits seeing black in those particular receptors).

16. Color Blindness
Color blindness tests to distinguish types of color blindness
Color blindness test to distinguish hues
Common color blindness test (Ishihara)
Article from Wall Street Journal on color blindness
Link to Ishishara history and color blindness facts
Link to “How Colorblindness Works”

17. How we see in the dark: Rods
Retinal cells that are very sensitive to light but only register shades of gray (i.e., no color)
Rods are located everywhere in the retina except in the fovea
Rods allow us to see at night without strong light – this is why we see less color at night
Because of where the rods are on the retina, we see best at night without light in the periphery of our vision
Dark adaptation
Rods are long, thin, cylindrical cells that are highly sensitive to light. They are concentrated in the sides of the retina and are active for black-and-white vision in dim light. Under impossibly ideal conditions, rods have the capacity to detect the light produced by one ten-billionth of a watt. On a perfectly clear, pitch-dark night, that’s like seeing the light of a match struck 30 miles away.
In chipmunks, pigeons, and other animals that are only active during the day, the retina contains no rods. Thus, they are virtually blind at night. Humans have about 120 million rods which means we can make out forms under low levels of illumination but can’t see as well at night as those animals that are nocturnal.
Often, we need to adjust to radical changes of illumination. You step into a dark movie theater after having been in the bright sunlight and you stumble around as if you were blind. After a few minutes, you can see again. This is dark adaptation: It takes about 20 minutes in darkness for your rods to kick in at full strength (though within about 30 seconds, you can see well enough to not walk right into a wall).

18. Rods & Cones
This is a photo of rods and cones and how they are attached to the back wall of the retina. It has been magnified to 14,000 times its original size.

19. Vision Electromagnetic energy
Long wavelengths: AC circuits, radio waves, infrared rays
Short wavelengths: visible light, X-rays, UV & gamma rays
Other animals can see other segments of the spectrum of electromagnetic energy
Bees can see ultraviolet rays and blue-violet, but not red
Pit vipers can see infrared rays
Dogs can’t see all the colors that humans can (no red)
You and I are visual creatures. How many times have you said ‘Show me.’ ‘I’ll believe it when I see it.’ ‘I saw it with my own eyes.’ ‘Out of sight, out of mind.’ Like other aspects of human anatomy, our visual system is a highly adapted product of evolution.
The eye is a highly evolved sensory receptor that has some really cool components. The eye is a concentration of cells that are sensitive to light energy. Your eye isn’t actually seeing color, but rather pulses of electromagnetic energy, which our eye then translates into experiencing color. The longer wavelengths include AC circuits, radio waves, and infrared rays. The shorter wavelengths include: visible light, UV rays, X-rays, and gamma rays. The visible light spectrum refers to only the part of the electromagnetic spectrum that humans can see. It ranges from 400 nanometers (violet) to 700 nanometers (red).
The eye is highly specialized by species such that animals are most adept at seeing those wavelengths that are most important to their survival. Humans have developed very sensitive acuity to the spectrum of wavelengths that is visible to us but other animals see other wavelengths. Bees don’t see red, but they do see ultraviolet rays and blue-violet light. This allows it to be able to better see pollen, their food source. The pit viper can see infrared rays (which humans are unable to see), which allow it to see better in the dark than its prey. Thus, it is more effective at hunting at night than its prey is at defending itself. And dogs don’t see all the colors that we see, but they do have better peripheral vision (though worse close-up vision).

20. Roy G. Biv This is the acronym most children use to learn
the colors of the visible light spectrum.

21. The Visible Light Spectrum
The length of a light wave determines its hue, or perceived color. To the human eye, white light is made up of all visible wavelengths combined. Short wavelengths look bluish, medium wavelengths look greenish, and long wavelengths look reddish.
A second property of light is its intensity, or amplitude, as measured by the height of the peaks in the wave. As wavelength determines color, amplitude determines brightness. The higher the amplitude, the brighter the light appears to be.
Transduction is the process where raw energy is converted into neural signals that are sent to the brain. These signals are then selected, organized, and interpreted through the perception stage.
Our Visible Spectrum
Transduction is the process where the eye converts electromagnetic energy (light) into nerve impulses.

22. Optic Nerve
From the receptor cells in the retina, the converted impulse from light is directed to the optic nerve, a large bundle of nerve fibers that carries impulses from the retina to the brain.
It sits on the retina but contains no cones or rods, so this is where you experience a ‘blind spot.’ (Click for a link to the blind spot test.)
We aren’t aware that we have a blind spot because our brain completes patterns that fall across our blind spot and because our eyes are constantly moving (‘filling’ it in)
Pg 149 in book - an activity to demonstrate this
The optic nerve is a pathway that carries visual information from each eyeball to the brain.
The area where the optic nerve enters the eye has no rods or cones, only axons. So each eye has a blind spot. You don’t normally notice it because your eyes are always moving, but you can find it using an exercise in your book.

23. The Blind Spot Test

24. Processing of Visual Information
Retina
Processes electrical impulses and starts to encode and analyze sensory information (at the most basic level into colors and shapes).
Optic Nerve
Neurons pick up the messages from retina, transmit to the thalamus, then on to the visual cortex, then on to more specific areas.
Basically, signals from the 130 million rods and cones are funneled through a mere 1 million axons in the optic nerve. Thus, the cells are integrating and compressing signals from multiple receptors. Axon fibers of ganglion cells form the optic nerve. The messages pass along the axons of the cells, to the optic chiasm, through the thalamus, and then on to the visual cortex (which you all know is located where??? That’s right, the occipital lobe!).

25. Thresholds and Stimulus Change
There is a minimum amount of stimulation for a sensation that has to be present for us to notice it.
The absolute threshold is the minimum amount of a stimulus necessary for us to notice it 50% of the time.
Example: A person with normal vision can detect the light of a single candle 30 miles away on a clear night.
Sensory adaptation
If a stimulus is unchanging, we become desensitized to it. What is an example of sensory adaptation?
We pay more attention to new stimuli because they are likely to be more significant..
A threshold is the minimum amount of any given stimulus that is needed for us to notice it. Beyond that, the absolute threshold is the minimum amount of any given stimulus that is needed for us to notice it half the time. Not every time, but half the time.
Our sensory systems are designed to detect novelty, contrast, and change – not sameness. After constant exposure to a stimulus, sensation fades. This decline in sensitivity is known as sensory adaptation. After a while, you simply get used to the new contact lenses in your eyes, the new watchband on your wrist, the noise level at work, or the coldness of winter. To those of you sensitive to the smells that often pervade the hallways of apartment buildings (or dorms), it is comforting to know that people also adjust to chronic odors. By adapting to a repeated stimulus, you are free to detect important changes in the environment.

26. Just Noticeable Difference
Just noticeable difference (JND)
Smallest difference in amount of stimulation that a specific sense can detect
Examples: How much does my classroom have to heat up before you realize the air is broken (again)? How much does the volume of your car stereo have to be turned up to distinguish the difference?
Sensory capacities are measured not only by our ability to detect low levels of stimulation, but also by the extent to which we can detect subtle differences. This ability is determined by asking subjects to compare the brightness of two bulbs, the loudness of two tones, or (as in my example earlier this year) the weight of two stacks of books, and so on. The subjects are given the two stimulus and asked to report whether they are the same or different. Or, the subject is asked, given one stimulation, to adjust the level of another stimulus so that the two are the same. Either way, it is possible to pinpoint the smallest change in stimulation that subjects can detect 50% of the time. This is the Just Noticeable Difference.
Now Weber noticed that JNDs increase with the size or intensity of the stimulus – and that the magnitude of a JND is a constant proportion of the original stimulus. In other words, as the stimulus increases in magnitude, a greater change is needed before it can be detected. For example, the JND for weight is about two percent. So if you lift a 50 ounce object and then a 51 ounce object, you will probably notice that the second one is heavier than the first. However, you would not feel a difference between one object that weighs 50 pounds and another that weighs 50 pounds, 1 ounce. Again, there is an absolute difference of one ounce; but a JND of 2 percent means that if your reference point is a 50-pound object, you wouldn’t detect a difference unless the second object is at least 2 percent heavier (or 51 pounds).
FYI: JND is 2% for brightness, 10% for loudness, and 20% for the taste of salt.

27. How We Perceive Images
Visual Processing

28. Why Sensation & Perception?
TN Psychology Standards 6.7 understand the relationship between the brain, sensory perceptions, and behavior. 6.9 analyze the role of perceptions in the psychological growth and development of humans.
OUR objectives:
To distinguish between sensation and perception using all five senses
To identify and explain how the parts of the body work together to create sensory experiences
To use our senses to explore our environments

29. Vision uses the additive process. Art uses the subtractive process.
An afterimage is a sensation that persists after prolonged exposure to a stimulus. What did you see before? In situations such as this one, staring at a green image leaves a red afterimage, yellow leaves a trace of blue and black leaves white.
Putting the pieces together, Hering proposed the opponent-process theory of color vision. According to this theory, there are three types of visual receptors, and each is sensitive to a pair of complementary or opponent colors. One type reacts to the colors blue and yellow, a second type detects red and green, and a third type detects variations in brightness ranging from black to white. Within each pair of red-green, blue-yellow, and black-white receptors, some parts fire more to one color whereas other parts react to its opposite. That’s why we never see bluish yellow or reddish green but we might see bluish green and reddish yellow. While seeing one color at a specific spot on the retina, you cannot also see its opposite color on the same spot.
Trichromatic theory argues that the eye has three kinds of color sensors (cones), with each sensor responding maximally to a distinct range of wavelength. Color vision arises from the combinations of neural impulses from these three different kinds of sensors. A lot of researchers believe that the corresponding wavelengths here map onto blue, green, and red. For example, in presenting a yellow (580 nm) stimulus to a participant, you would most likely see a large amount of firing of the green (530 nm) and red (650 nm) cones, with little to no firing of the blue (460 nm) cones.
Other researchers argued that some color combinations don’t make sense – red and green mixed together don’t make yellow; it looks more like mud. But, in addition to the fact that mixing physical colors before the wavelengths get to the retina is a different process than the cones firing when the resulting wavelength reaches the retina, sensing color on the retina from various wavelengths then gets sent to the brain for perception via opponent-process cells in the brain. Thus, the opponent-process theory was offered as a complimentary theory to the trichromatic theory. This theory states that if a color is present, it causes cells that register it to inhibit the perception of an opposing color.
The opponent-process theory proposes six psychologically primary colors, which are assigned by pairs to three kinds of receptors: white-black receptors, red-green receptors, and yellow-blue receptors. If white is present, then the perception of black is inhibited, if red is present then green is inhibited, and if yellow is present then blue is inhibited.
Inhibitions, along with various mixtures of these primary light wavelengths, more readily explain the complex range of colors that we experience after sensations of colors are sent from the retina to the brain’s opponent-process receptor cells.
To clarify:
Trichromatic theory and opponent-process theory work at two different levels, specifying different receptor cells at different stages in the process.
When you are looking at black words on a white page, you see both black and white because you are stimulating many white-black receptors on your retina. You see a black letter with one white-black receptor (and doing so inhibits seeing white in that particular receptor) and you see the background white with other white-black receptors (and doing so inhibits seeing black in those particular receptors).
Vision uses the additive process. Art uses the subtractive process.
The primary colors of light are red, green, and blue; art’s primary colors are red, yellow, and blue.

31. Opponent Process Theory
The picture of the American flag illustrates an afterimage.
When you stared at the flag, you fatigued your green-detection neurons.
When you stared at the white space, your red-detection neurons took over, creating the afterimage of the flag. The yellow and black replace the blue and white.

32. Gestalt Principles of Vision
Figure-ground
We recognize figures (objects) by distinguishing them from the background
Proximity
Marks that are near one another tend to be grouped together
Okay, so sensory intake of visual stimuli must be processed by the brain into organized structures (the perception side of the equation). We don’t see just random splashes of color, unless you’re looking at a Jackson Pollack, you see organized structures that have shape and meaning.
The eye by itself cannot organize visual input into shapes that correspond to objects – that’s the brain’s job!
There are some Gestalt principles that help organize this process, and are common concepts for us to rely on as we are organizing this information.
The figure-ground principle is that people automatically focus on some objects in the perceptual field to the exclusion of others. What we focus on is called the figure. Everything else fades into the ground. So, a teacher standing in front of a blackboard, the printed black words on a page, the lights on the car coming at us on a dark highway, a scream in the night, and the lead signer’s voice in a rock band are all common figures and grounds. Gestalt psychologists are quick to point out that these perceptions are in the eyes (or ears) of the beholder – but that we are also prone to ‘figurize’ objects that are close to us, novel, intense, loud, and moving rather than still. But as the image at the bottom of the page shows, we are mentally capable of flip-flopping the figure-ground perspective.
The proximity principle holds that the closer objects are to another, the more likely they are to be perceived as a unit. The lines at the bottom of the page are more likely to be perceived as rows rather than as columns because they are nearer to one another horizontally than vertically.

33. Gestalt Principles of Vision
Closure
We tend to fill in gaps in a figure
Similarity
Marks that look alike tend to be grouped together
The closure principle holds that where there are gaps in a pattern that resembles a familiar form, people mentally ‘close’ the gaps and perceive the object as whole. This tendency enables us to recognize imperfect representations in hand drawings, written material, and so on.
The similarity principle holds that objects that are similar in shape, size, color, or any other feature tend to be grouped together. So the dots form perceptual columns rather than rows because of the similarities in color.

34. Gestalt Principles of Vision
Continuity
Marks that tend to fall along a smooth curve or a straight line tend to be grouped together
The continuity principle holds that people perceive the contours of straight and curved lines as continuous flowing patterns. In this figure, we see points 1 and 2 as belonging to one line and points 3 and 4 as belonging to a second line. The same pattern could be seen as to V shapes, but instead we perceive two smooth lines that form a cross in the center.

35. Depth Perception
How is it that we perceive a 3-dimensional world when our eyes only project a 2-dimensional image on our retinas??!
Our brain uses different cues to perceive depth.
We use binocular (both eyes) and monocular (one eye) cues to determine depth.
So how do we know that objects in three-dimensional space have depth, and how do we perceive distance when images projected on each retina are flat and only two-dimensional? There are two types of information that are used in depth perception: binocular cues and monocular cues.
The main binocular cue is that of binocular disparity. With our eyes being set about 2 ½ inches away from each other, each retina receives a slightly different image of the world.
To demonstrate, hold your finger about 4 inches from your nose and shut your right eye. Then shut only your left eye and look at the finger. Right. Left. As you switch back and forth, you’ll see that each eye picks up the image from a slightly different vantage point. If you hold your finger farther away and repeat this performance, you’ll notice that there is less image shifting.
The reason: Binocular disparity decreases with distance. Special neurons located in the visual cortex use this retinal information to calculate depth, distance, and dimensionality.
You’ve probably all seen those posters that have the stereogram images where if you stare at them long enough, you can see a 3-D image ‘pop’ out. There are ways to alter images so that a 2-D image becomes 3-D…this was discovered in the 1800s and has led to the development of virtual reality technology.

36. Depth Perception: Binocular Cues
Retinal disparity
Since we use both our eyes to focus on an image, the angles used by each eye to put the image on the fovea of our retina is used by the brain to perceive distance.
Example: Using a View-Master produces what appears to be one image. Instead, it’s two image several feet apart with different angles. It projects the image taken by the left camera to the left eye and the image taken from the right camera to the right eye. Your brain combines the view to calculate distance and add depth.
Convergence
Convergence is a binocular depth cue related to the tension in the eye muscles when eyes track inward to focus on objects close to the viewer.
There are cues within the stimulus itself that provide clues to our brain about depth perception, which don’t rely on the use of both our eyes.
For example, the motion of a stimulus can specify the distance of the stimulus. There is a phenomenon known as motion parallax where as we move, objects that are actually standing still appear to move. If you’ve ever stared out the window of a car as you travel down the highway, you notice that closer objects that you are not focusing on seem to whiz right past you while farther away objects that you focus on seem to move in the same direction as you. Objects at different distances also appear to move at different speeds. There is another motion-based illusion known as illusory reverse-motion where if you are in a stable object (say a car) another object to the side (say another car in the next lane over) moves forward, you may actually have a perception that you are moving backward. I drive a manual transmission car and so don’t always need to have my foot on the brake to stay in one spot…needless to say, when this has happened to me, I immediately throw my foot on the brake only to realize that I wasn’t moving at all!!
We can also get cues from a stimulus based on its texture gradient. These are progressive changes in the texture of the object that signal changes in distance. As a collection of objects recedes into the horizon, they appear to be spaced more closely together, which makes the surface texture appear to become denser.
The last monocular cue that we are going to discuss is that of linear perspective. With distance, the parallel contours of highways, rivers, railroad tracks, and other row-like structures perceptually converge – and eventually they reach a vanishing point. The more the lines converge, the greater the perceived distance.

37. Retinal Disparity

38. Depth Perception: Monocular Cues
Our brain also uses information from the stimulus that does not involve our use of both eyes
Relative size
If an object of known size appears large, it is probably close.
If an object of known size appears small, it is probably distant.
Texture gradient
Progressive changes in surface texture that signal distance
Linear perspective
Parallel objects seem to get closer together as they get farther away
There are cues within the stimulus itself that provide clues to our brain about depth perception, which don’t rely on the use of both our eyes.
For example, the motion of a stimulus can specify the distance of the stimulus. There is a phenomenon known as motion parallax where as we move, objects that are actually standing still appear to move. If you’ve ever stared out the window of a car as you travel down the highway, you notice that closer objects that you are not focusing on seem to whiz right past you while farther away objects that you focus on seem to move in the same direction as you. Objects at different distances also appear to move at different speeds. There is another motion-based illusion known as illusory reverse-motion where if you are in a stable object (say a car) another object to the side (say another car in the next lane over) moves forward, you may actually have a perception that you are moving backward. I drive a manual transmission car and so don’t always need to have my foot on the brake to stay in one spot…needless to say, when this has happened to me, I immediately throw my foot on the brake only to realize that I wasn’t moving at all!!
We can also get cues from a stimulus based on its texture gradient. These are progressive changes in the texture of the object that signal changes in distance. As a collection of objects recedes into the horizon, they appear to be spaced more closely together, which makes the surface texture appear to become denser.
The last monocular cue that we are going to discuss is that of linear perspective. With distance, the parallel contours of highways, rivers, railroad tracks, and other row-like structures perceptually converge – and eventually they reach a vanishing point. The more the lines converge, the greater the perceived distance.

39. Depth Perception: Monocular Cues
Relative motion
Perceived slowness indicates that an object is distant.
Interposition
Closer objects obstruct the view of more distant objects.
Relative height
Distant objects appear relatively higher in your field of vision than close objects do.
Relative clarity
Distant objects appear less clear than nearby objects.
There are cues within the stimulus itself that provide clues to our brain about depth perception, which don’t rely on the use of both our eyes.
For example, the motion of a stimulus can specify the distance of the stimulus. There is a phenomenon known as motion parallax where as we move, objects that are actually standing still appear to move. If you’ve ever stared out the window of a car as you travel down the highway, you notice that closer objects that you are not focusing on seem to whiz right past you while farther away objects that you focus on seem to move in the same direction as you. Objects at different distances also appear to move at different speeds. There is another motion-based illusion known as illusory reverse-motion where if you are in a stable object (say a car) another object to the side (say another car in the next lane over) moves forward, you may actually have a perception that you are moving backward. I drive a manual transmission car and so don’t always need to have my foot on the brake to stay in one spot…needless to say, when this has happened to me, I immediately throw my foot on the brake only to realize that I wasn’t moving at all!!
We can also get cues from a stimulus based on its texture gradient. These are progressive changes in the texture of the object that signal changes in distance. As a collection of objects recedes into the horizon, they appear to be spaced more closely together, which makes the surface texture appear to become denser.
The last monocular cue that we are going to discuss is that of linear perspective. With distance, the parallel contours of highways, rivers, railroad tracks, and other row-like structures perceptually converge – and eventually they reach a vanishing point. The more the lines converge, the greater the perceived distance.

40. Relative Size

41. Texture Gradient

42. Linear Perspective

43. Relative motion

44. Interposition

45. Relative Height

46. Relative Clarity

47. Perceptual Constancy - Vision
The image of an object on your retina may vary in size, shape, and brightness, but we SEE it as constant.
Size constancy
The tendency to view an object as constant in size despite changes in the size of its image on the retina (as we move)
Shape constancy
The tendency to see an object as retaining its form despite changes in orientation
Lightness constancy
The tendency to see an object as having a constant level of lighting no matter how the lighting conditions change
Size constancy: If you see a car from a block away, you still perceive its size not to change even though the image is smaller on your retina compared to a car parked right next to you.
Shape constancy: You perceive a book to maintain its rectangular shape even though at the different angles, your retinal image of the book is not a rectangle.

48. Size Constancy

49. Shape Constancy The tendency to see an object as retaining its form
despite changes in orientation

50. Lightness Constancy
Squares A and B are the exact same shade of gray. What gives us the illusion that they are different shades of gray?

51. Perceptual Set
The “power of suggestion” produced by perceptual set influences us in many ways. Six times as many preschool children think French fries taste better when they are served in a McDonald’s bag instead of a plain white bag (Thinking about Psychology, 3rd edition, p. 131).
When you cover the side pictures, what do you see? What do you see when they are uncovered? Perceptual set is a mental predisposition to perceive one way and not another.

52. Müller-Lyer Illusion A B C
The Müller-Lyer illusion has two forms. Above, you see one version. Which is longer? AB? Or BC? To the right, you see another version.

53. Ames Room Illusion
The Ames Room illusion makes these cards appear different sizes even though they are identical in size. Click on the picture for an explanation.

54. Click on the grid for an explanation of the illusion.
Hermann Grid Illusion
Click on the grid for an explanation of the illusion.

55. TriBar Illusion (Click on the title for a “how to” demo.)
The Tribar Illusion from one angle
The Tribar Illusion from another angle

56. Change Blindness
“Psychologists who study the fascinating phenomenon of change blindness know that merely looking at something is not the same as actively paying attention to it. As the demonstration in this video shows, people can be blind to significant changes in a visual scene that are obvious to someone who expects that these changes are going to happen” (NOVA, 2011).

57. Top-Down Processing
Top-down processing is the use of preexisting knowledge to organize individual features into a unified whole. This is also a form of perceptual set (expectancy)
You use top-down processing when you make a puzzle using a picture of the finished puzzle as your guide.
What is another example of top-down processing?

58. Bottom-Up Processing
Bottom-up processing is taking the smaller feature and building to a complete perception from the small pieces.
You use bottom-up processing when you build a puzzle without the box top.
What is another example of bottom-up processing?
Which form of processing would you consider Gestalt? Think about what a Gestalt is.